The long-term objective of Project 3 is to characterize the natural history and developmental epidemiology of sexually transmitted infections (STI) among young men from middle to late adolescence and into young adulthood. The,specific aims are: (1) To describe adolescent men's patterns of infection by 5 sexually transmitted organisms (C. trachomatis trachomatis. Neisseria gonorrhoeae. Trichomonas vaginalis. Mycoplasma genitalium, Ureaplasma urealyticum, biovar 2), and non-gonococcal urethritis as a function of time;(2) To describe adolescent men's patterns of infection acquisition, organism shedding, symptom development, and pyuria for each target organism using event-contingent every other day samples for 14 days;And (3) To describe the psychological, interpersonal, social and behavioral phenomena associated with occurrence of sexual events using electronic cell phone diary reports among adolescent men. Through the Clinical Core, we will recruit 234 African American, Latino and Euro-American young men, ages 14-17 at enrollment, from a community with high STI prevalence. Participants will be followed for up to 57 months. In conjunction with the Laboratory Core, we will test monthly surveillance urine samples and more frequent event-contingent samples with PCR and cell counts to characterize the natural history ofthe above 5 STI and NGU across adolescence. Quarterly audio computer assisted self interviews (ACASI) will be used to characterize demographic, behavioral, biological, and socio-cultural correlates of STI acquisition and sequence. Daily cell phone diaries will be used to describe the psychological, interpersonal, and behavioral phenomena associated with specific sexual events and STI. The proposed research addressed three priorities related to STI: (1) A focus on adolescents, (2) The interactions among different organisms, including emerging pathogens such as M. genitalium and (3) the provision of key developmental epidemiology and behavioral data to inform interventions.